Is there any way to install a version of the game that is unpatched without using the original disk again, such as depatching of some sort? I've somehow managed to misplace the original disk, as has someone else in a game being started elsewhere, and this is necessary to play it.

None that I can think of.
I suppose if you find an earlier patch you could simply run that. It might patch-backward all of the files involved. But even then, there would be no way to get back to the disk original version without having a copy of that version.

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